Cross talk reduction for high dynamic range image sensors

ABSTRACT

A multi-color HDR image sensor includes at least a first combination color pixel with a first color filter and an adjacent second combination color pixel with a second color filter which is different from the first color filter, wherein each combination color pixel includes at least two sub-pixels having at least two adjacent photodiodes. Within each combination color pixel, there is a dielectric deep trench isolation (d-DTI) structure to isolate the two adjacent photodiodes of the two adjacent sub-pixels with same color filters in order to prevent the electrical cross talk. Between two adjacent combination color pixels with different color filters, there is a hybrid deep trench isolation (h-DTI) structure to isolate two adjacent photodiodes of two adjacent sub-pixels with different color filters in order to prevent both optical and electrical cross talk. Each combination color pixel is enclosed on all sides by the hybrid deep trench isolation (h-DTI) structure.

REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.15/478,085 filed Apr. 3, 2017, now pending. U.S. patent application Ser.No. 15/478,085 is hereby incorporated by reference.

TECHNICAL FIELD

This disclosure relates generally to semiconductor image sensors, and inparticular but not exclusively, relates to the pixel isolationstructures in high dynamic range image sensors.

BACKGROUND INFORMATION

Image sensors have become ubiquitous. They are widely used in digitalstill cameras, cellular phones, security cameras, as well as medical,automobile and other applications. The typical image sensor operates asfollows. Image light from an external scene is incident on the imagesensor. The image sensor includes a plurality of photosensitive elementssuch that each photosensitive element absorbs a portion of the incidentimage light. Each photosensitive element included in the image sensor,such as photodiodes, generates image charges upon absorption of theimage light. The amount of generated image charges is proportional tothe intensity of the image light. The generated image charges may beused to produce an image representing the external scene.

The device architecture of image sensors has continued to advance at agreat pace due to increasing demands for higher resolution, lower powerconsumption, increased dynamic range, etc. These demands have alsoencouraged the further miniaturization and integration of image sensorsinto these devices. For high dynamic range image sensors, combinationpixels are usually used to sense individual electromagnetic radiationwavelength in order to accommodate a wide range of lighting situations.For example, in a combination red pixel, one sub-pixel can be used tosense bright red light conditions while another sub-pixel can be used tosense low red light conditions. The miniaturization of image sensors mayresult in a decreased distance between neighboring photosensitiveelements. As the distance between photosensitive elements decreases, thelikelihood and magnitude of optical and electrical crosstalk betweenphotosensitive elements may increase.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive examples of the invention are describedwith reference to the following figures, wherein like reference numeralsrefer to like parts throughout the various views unless otherwisespecified.

FIG. 1 is a block diagram schematically illustrating one example of ahigh dynamic range (“HDR”) imaging system, in accordance with anembodiment of the disclosure.

FIG. 2 is a plan view illustration of a multi-color HDR image sensorthat includes four combination color pixels that each pixel includesfour HDR sub-pixels having four photodiodes isolated by dielectric deeptrench isolation (d-DTI) structures.

FIG. 3 is a plan view illustration of a multi-color HDR image sensorthat includes four combination color pixels that each pixel includesfour HDR sub-pixel having four photodiodes isolated by both hybrid deeptrench isolation (h-DTI) structures and d-DTI structures, in accordancewith an embodiment of the disclosure.

FIG. 4 is a magnified cross-section illustration of an example imagesensor in FIG. 3 along A-A′ direction, in accordance with an embodimentof the disclosure.

FIG. 5A-5D illustrate an example fabrication method of the image sensorin FIG. 4, in accordance with an embodiment of the disclosure.

Corresponding reference characters indicate corresponding componentsthroughout the several views of the drawings. Skilled artisans willappreciate that elements in the figures are illustrated for simplicityand clarity and have not necessarily been drawn to scale. For example,the dimensions of some of the elements in the figures may be exaggeratedrelative to other elements to help to improve understanding of variousembodiments of the present invention. Also, common but well-understoodelements that are useful or necessary in a commercially feasibleembodiment are often not depicted in order to facilitate a lessobstructed view of these various embodiments of the present invention.

DETAILED DESCRIPTION

Examples of an apparatus and method for an image sensor with both hybriddeep trench isolation structures and dielectric deep trench isolationstructures are described herein. In the following description, numerousspecific details are set forth to provide a thorough understanding ofthe examples. However, one skilled in the relevant art will recognizethat the techniques described herein can be practiced without one ormore of the specific details, or with other methods, components,materials, etc. In other instances, well-known structures, materials, oroperations are not shown or described in details in order to avoidobscuring certain aspects.

Reference throughout this specification to “one example” or “oneembodiment” means that a particular feature, structure, orcharacteristic described in connection with the example is included inat least one example of the present invention. Thus, the appearances ofthe phrases “in one example” or “in one embodiment” in various placesthroughout this specification are not necessarily all referring to thesame example. Furthermore, the particular features, structures, orcharacteristics may be combined in any suitable manner in one or moreexamples.

Throughout this specification, several terms of art are used. Theseterms are to take on their ordinary meaning in the art from which theycome, unless specifically defined herein or the context of their usewould clearly suggest otherwise. It should be noted that element namesand symbols may be used interchangeably through this document (e.g., Sivs. silicon); however, both have identical meaning.

FIG. 1 is a block diagram illustrating one example of imaging system100. Imaging system 100 includes pixel array 104, control circuitry 103,readout circuitry 101, and function logic 102. In one example, pixelarray 104 is a two-dimensional (2D) array of photodiodes, or imagesensor pixels (e.g., pixels P1, P2 . . . , Pn). As illustrated,photodiodes are arranged into rows (e.g., rows R1 to Ry) and columns(e.g., column C1 to Cx) to acquire image data of a person, place,object, etc., which can then be used to render a 2D image of the person,place, object, etc. However, in other examples, it is appreciated thatthe photodiodes do not have to be arranged into rows and columns and maytake other configurations.

In one example, after the image sensor photodiode/pixel in pixel array104 has acquired its image data or image charge, the image data isreadout by readout circuitry 101 and then transferred to functionallogic 102. In various examples, readout circuitry 101 may includeamplification circuitry, analog-to-digital (ADC) conversion circuitry,or otherwise. Function logic 102 may simply store the image data or evenmanipulate the image data by applying post image effects (e.g., crop,rotate, remove red eye, adjust brightness, adjust contrast, orotherwise). In one example, readout circuitry 101 may read out a row ofimage data at a time along readout column lines (illustrated) or mayreadout the image data using a variety of other techniques (notillustrated), such as a serial readout or a full parallel readout of allpixels simultaneously.

In one example, control circuitry 103 is coupled to pixel array 104 tocontrol operation of the plurality of photodiodes in pixel array 104.For example, control circuitry 103 may generate a shutter signal forcontrolling image acquisition. In one example, the shutter signal is aglobal shutter signal for simultaneously enabling all pixels withinpixel array 104 to simultaneously capture their respective image dataduring a single acquisition window. In another example, the shuttersignal is a rolling shutter signal such that each row, column, or groupof pixels is sequentially enabled during consecutive acquisitionwindows. In another example, image acquisition is synchronized withlighting effects such as a flash.

In one example, imaging system 100 may be included in a digital camera,cell phone, laptop computer, automobile or the like. Additionally,imaging system 100 may be coupled to other pieces of hardware such as aprocessor (general purpose or otherwise), memory elements, output (USBport, wireless transmitter, HDMI port, etc.), lighting/flash, electricalinput (keyboard, touch display, track pad, mouse, microphone, etc.),and/or display. Other pieces of hardware may deliver instructions toimaging system 100, extract image data from imaging system 100, ormanipulate image data supplied by imaging system 100.

FIG. 2 is a plan view illustration of a multi-color HDR image sensor 200that includes four combination color pixels that each pixel includesfour HDR sub-pixels having four photodiodes. The four combination colorpixels include a combination red pixel 202, a first combination greenpixel 203, a second combination green pixel 204, and a combination bluepixel 205. Each combination color pixel includes four HDR sub-pixelsthat include 4 photodiodes sharing a common floating diffusion at thecenter of the color pixel, wherein each photodiode has an individualtransfer gate. As one example, the combination red pixel 202 has fourphotodiode 202 a, 202 b, 202 c and 202 d. The photodiode 202 a has atransfer gate 202 aa, the photodiode 202 b has a transfer gate 202 bb,the photodiode 202 c has a transfer gate 202 cc, and the photodiode 202d has a transfer gate 202 dd. The photodiode 202 a, 202 b, 202 c and 202d share a common floating diffusion 202 e at the center of thecombination red pixel 202. There is a standard d-DTI structure 201 toisolate the adjacent photodiodes in order to prevent the electricalcross talk. However, such d-DTI structure 201 may not fully block theoptical cross talk between the adjacent sub-pixel photosensitiveelements with different color filters, such as the optical cross talkbetween 202 b and 203 a, 202 d and 203 c, 204 b and 205 a, or 204 d and205 c. It is desirable to improve the isolation structures betweenadjacent sub-pixels with different color filters in order to reduce notonly electrical cross talk but also optical cross talk for betterimaging resolution.

FIG. 3 is a plan view illustration of a multi-color HDR image sensor 300that includes four combination color pixels that each pixel includesfour HDR sub-pixels having four photodiodes isolated by both h-DTIstructures and d-DTI structures, in accordance with an embodiment of thedisclosure. In one example, the four combination color pixels include acombination red pixel 302, a combination blue pixel 303, a combinationgreen pixel 304, and a combination IR pixel 305. The four combinationcolor pixels may also include combination secondary primary color(magenta, yellow and cyan) pixels, combination black pixels andcombination white (or clear) pixels. The adjacent combination colorpixels could be the same combination color pixels or differentcombination color pixels. Each combination color pixel includes four HDRsub-pixels that include 4 photodiodes sharing a common floatingdiffusion at the center of the combination color pixel, and eachphotodiode has its own transfer gate. Each of the HDR sub-pixels mayhave the same physical configurations and electrical circuitconfigurations. The HDR sub-pixels may also have the different physicalconfigurations and electrical circuit configurations. Within eachcombination color pixel, there is a d-DTI structure 301 a to isolate twoadjacent photodiodes of two adjacent sub-pixels with same color filtersin order to prevent the electrical cross talk. Between two adjacentcombination color pixels with different color filters, there is a h-DTIstructure 301 b to isolate two adjacent photodiodes of two adjacentsub-pixels with different color filters in order to prevent both opticaland electrical cross talk. In one example, each combination color pixelis enclosed on all sides by the h-DTI structure 301 b.

In one example demonstrated in FIG. 3, the combination red pixel 302 hasfour photodiode 302 a, 302 b, 302 c and 302 d. The photodiode 302 a hasa transfer gate 302 aa, the photodiode 302 b has a transfer gate 302 bb,the photodiode 302 c has a transfer gate 302 cc, and the photodiode 302d has a transfer gate 302 dd. The photodiode 302 a, 302 b, 302 c and 302d share a common floating diffusion 302 e at the center of thecombination red pixel 302. The combination green pixel 304 has fourphotodiode 304 a, 304 b, 304 c and 304 d. The photodiode 304 a has atransfer gate 304 aa, the photodiode 304 b has a transfer gate 304 bb,the photodiode 304 c has a transfer gate 304 cc, and the photodiode 304d has a transfer gate 304 dd. The photodiode 304 a, 304 b, 304 c and 304d share a common floating diffusion 304 e at the center of thecombination green pixel 304. Within the combination red pixel 302, thed-DTI structure 301 a isolates the photodiode 302 a from the photodiode302 b, the photodiode 302 c from the photodiode 302 d, the photodiode302 a from the photodiode 302 c, and the photodiode 302 b from thephotodiode 302 d. Within the combination green pixel 304, the d-DTIstructure 301 a isolates the photodiode 304 a from the photodiode 304 b,the photodiode 304 c from the photodiode 304 d, the photodiode 304 afrom the photodiode 304 c, and the photodiode 304 b from the photodiode304 d. The combination red pixel 302 is adjacent to the combinationgreen pixel 304. Between the combination red pixel 302 and thecombination green pixel 304, there is the h-DTI structure 301 b toisolate the photodiode 302 c from the adjacent photodiode 304 a, and thephotodiode 302 d from the adjacent photodiode 304 b. Both thecombination red pixel 302 and the combination green pixel 304 areenclosed on all sides by the h-DTI structure 301 b.

FIG. 4 is a cross-section illustration of an example image sensor 400along A-A′ direction in FIG. 3, in accordance with an embodiment of thedisclosure. The image sensor 400 includes a semiconductor material 410with a first side 414 as the backside of semiconductor material 410 anda second side 408 as the front side of semiconductor material 410. Onthe first side 414, there is dielectric material 415, a plurality ofmetal grids 416, a plurality of color filters 402 and 404, and aplurality of microlenses 418. On the second side 408, there are aplurality of transfer gates 419 and dielectric material 407. In thesemiconductor material 410, there are a plurality of photodiodes 401 a,401 b, 401 c and 401 d which may have same or different physicalconfigurations, a plurality of shallow trench isolation (STI) structures409 extending from the second side 408 toward the first side 414, aplurality of deep isolation wells 411 disposed between the first side414 and the second side 408, a plurality of d-DTI structures 417extending from the first side 414 toward the second side 408, and aplurality of h-DTI structures 420 extending from the first side 414toward the second side 408.

In the illustrated example, the adjacent photodiodes are separated fromeach other by the deep isolation well 411. A first deep isolation well411 a, which is disposed between two adjacent photodiodes with differentcolor filters, includes one respective optically aligned h-DTI structure420 and one STI structure 409 with respect to incident light that isnormal to the first side 414 of the semiconductor material 410. A seconddeep isolation well 411 b, which is disposed between two adjacentphotodiodes with same color filters, includes one respective opticallyaligned d-DTI structure 417 and one STI structure 409 with respect toincident light that is normal to the first side 414 of the semiconductormaterial 410.

In the illustrated example in FIG. 4, each of the d-DTI structures 417includes a dielectric material only and extends from the first side 414toward the second side 408 of the semiconductor material 410. Each ofthe h-DTI structures 420 includes a shallow portion 413 and a deepportion 412. The shallow portion 413 extends from the first side 414toward the second side 408 of the semiconductor material 410. Theshallow portion 413 includes a dielectric material region 413 a and ametal region 413 b such that at least part of the dielectric materialregion 413 a is disposed between the metal region 413 b and thesemiconductor material 410. The deep portion 412 extends from theshallow portion 413 and is disposed between the shallow portion 413 andthe second side 408 of the semiconductor material 410. The deep portion412 may include dielectric material region 412 a which may have the sameor different dielectric materials in the dielectric material region 413a. The dielectric material region 412 a and 413 a may include one typeof dielectric materials. The dielectric material region 412 a and 413 amay also include multilayers formed with different types of dielectricmaterials, wherein each layer may have a different type of dielectricmaterials including at least one positive charge dielectric material orone negative charge dielectric material. The layer abut to thesemiconductor material 410 may have the highest dielectric constantamong all the dielectric materials used. In one example, the multilayermay include a SiO2 layer and a high-k material layer between the SiO2layer and the semiconductor material 410.

In some examples, the metal region 413 b may include any one of thegroup comprising of W, Al, Cu, Ag, Au, Ti, Ta, Pb, and Pt. Thedielectric materials in both d-DTI structures 417 and h-DTI structures420 may include oxides/nitrides such as silicon oxide (SiO₂), hafniumoxide (HfO₂), silicon nitride (Si₃N₄), silicon oxynitirde(SiO_(x)N_(y)), tantalum oxide (Ta₂O₅), titanium oxide (TiO₂), zirconiumoxide (ZrO₂), aluminum oxide (Al₂O₃), lanthanum oxide (La₂O₃),praseodymium oxide (Pr₂O₃), cerium oxide (CeO₂), neodymium oxide(Nd₂O₃), promethium oxide (Pm₂O₃), samarium oxide (Sm₂O₃), europiumoxide (Eu₂O₃), gadolinium oxide (Gd₂O₃), terbium oxide (Tb₂O₃),dysprosium oxide (Dy₂O₃), holmium oxide (Ho₂O₃), erbium oxide (Er₂O₃),thulium oxide (Tm₂O₃), ytterbium oxide (Yb₂O₃), lutetium oxide (Lu₂O₃),yttrium oxide (Y₂O₃), or the like. Additionally, one skilled in therelevant art will recognize that any stoichiometric combination of theabove metals/semiconductors and their oxides/nitrides/oxynitrides may beused, in accordance with the teachings of the present invention.

The magnitude of electrical crosstalk between adjacent photodiodes maybe reduced by electrically isolating individual photodiodes. Thedielectric material region 413 a in the shallow portion 413 and thedielectric material region 412 a in the deep portion 412 of eachindividual h-DTI structures 420 may, at least partially, electricallyisolate the adjacent photodiodes optically aligned with different colorfilters which are disposed proximate to the first side 414 ofsemiconductor material 410. The metal region 413 b in the shallowportion 413 is also electrically isolated by the dielectric materialregion 413 a from individual photodiodes. The adjacent photodiodes withsame color filters may be, at least partially, electrically isolated byeach individual d-DTI structure 417.

The magnitude of optical crosstalk between adjacent photodiodes withdifferent color filters in the plurality of photodiodes 401 a, 401 b,401 c, and 401 d may be reduced by the metal region 413 b in eachindividual h-DTI structures 420. The metal region 413 b may absorb,reflect, or refract incident light such that optical crosstalk isminimized. In one example, at least part of the metal region 413 b iswider than the deep portion 412 of the h-DTI structure 420. Asillustrated, the metal region 413 b of the individual h-DTI structures420 may taper from the first side 414 of the semiconductor material 410towards the deep portion 412. The amount of taper for the metal region413 b may be designed such that off-axis incident light propagatesthrough the first side 414 of the semiconductor material 410 and isreflected by the metal region 413 b towards each of the plurality ofphotodiodes 401 a, 401 b, 401 c and 401 d.

In one example which is illustrated in FIG. 0.4, there is a tradeoff touse either the h-DTI structures 420 or the d-DTI structures 417 toisolate the adjacent photodiodes. For the h-DTI structures 420, themetal material such as W in the metal region 413 b may absorb theincident light in order to minimize the optical cross talk, however, thelight absorption also degrades the sensitivity of the image sensor. Forthe d-DTI structures 417, the dielectric materials 417 a in the deeptrench may not be able to absorb, reflect, or refract incident light inorder to minimize the optical crosstalk. In order to maintain thesensitivity of the image sensors as well as reduce the optical andelectrical cross talk, the h-DTI structures 420 are placed only betweentwo adjacent photodiodes with the different color filters, whereas thed-DTI structures 417 are placed only between two adjacent photodiodeswith the same color filters. In one example, the photodiode 401 b withthe red color filter 402 is separated from the photodiode 401 c with thegreen color filter 404 by the h-DTI structure 420, and the photodiode401 a with the red color filter 402 is separated from the photodiode 401b also with the red color filter by the d-DTI structure 417. In onefurther example which is illustrated in FIG. 3, each of the combinationmulti-color pixels 202, 203, 204 and 205 are surrounded on all sides bythe h-DTI structures 301, and within each of the combination multi-colorpixels, there is only d-DTI structures 302 to separate the adjacentsub-pixels.

FIGS. 5A-5D illustrates an example method 500 for fabrication of animage sensor in FIG. 4. The order in which some or all of FIGS. 5A-5Dappear in method 500 should not be deemed limiting. Rather, one ofordinary skill in the art having the benefit of the present disclosurewill understand that some of method 500 may be executed in a variety oforders not illustrated, or even in parallel. Furthermore, method 500 mayomit certain process steps and figures in order to avoid obscuringcertain aspects. Alternatively, method 500 may include additionalprocess steps and figures that may not be necessary in someembodiments/examples of the disclosure.

FIG. 5A illustrates a semiconductor material 504 having a first side 521opposite a second side 520. In one example, semiconductor material 504is silicon. A plurality of photodiodes 501 a, 501 b, 501 c, and 501 dare disposed in the semiconductor material 504 between the first side521 and the second side 520. In one example, the plurality ofphotodiodes is formed by ion implantation. A plurality of deep isolationwells 502 are disposed in the semiconductor material 504. Eachindividual deep isolation well 502 may extend from the first side 521 tothe second side 520 of semiconductor material 504. In one example,individual photodiodes 501 a, 501 b, 501 c, and 501 d are disposedbetween individual deep isolation wells 502. In one example, theplurality of deep isolation wells 502 are formed by ion implantation. Aplurality of first trenches 503 are etched that extend from the firstside 521 towards the second side 520 of semiconductor material 504. Inone example, each individual first trench 503 is etched withinindividual deep isolation well 502 such that each individual firsttrench 503 is disposed within a corresponding deep isolation well 502.

FIG. 5B illustrates a step of selectively widening a shallow portion 505in some of the plurality of first trenches 503 to form a plurality ofsecond trenches 503 a proximate to the first side 521 of thesemiconductor material 504, wherein each of the plurality of the secondtrenches 503 a with the widened shallow portion 505 is disposed betweentwo adjacent photodiodes with different color filters which will bedisposed in the subsequent steps after the step illustrated in FIG. 5D(not illustrated in FIG. 5A-5D).

In one example, the photodiode 501 a and 501 b will optically align withred color filters, and the photodiode 501 c and 501 d will opticallyalign with green color filters. The second trench 503 a is disposedbetween the photodiode 501 b and 501 c. The first trench 503 is disposedbetween the photodiode 501 a and 501 b.

In one example, a deep portion 506 in the plurality of the secondtrenches 503 a is disposed between the shallow portion 505 and thesecond side 520 of the semiconductor material 504. In one example, theshallow portion 505 tapers from the first side 521 towards the secondside 520 of the semiconductor material 504 such that a width of theshallow portion 505 proximate to the first side 521 is greater than thewidth of the deep portion 506 proximate to the second side 520.

FIG. 5C illustrates depositing a dielectric material 508 within theplurality of first trenches 503 and second trenches 503 a. In oneexample, the plurality of first trenches 503 are completely filled bythe dielectric material 508. On the other hand, the deep portion 506 inthe plurality of second trenches 503 a is also completely filled by thedielectric material 508. The shallow portion 505 of a plurality ofsecond trenches 503 a is partially filled by the dielectric material508, which is disposed on the sidewalls of the shallow portion 505 ofsecond trenches 503 a. An empty space 507 is formed in the middle of theshallow portion 505 after the dielectric material 508 is deposited asshown in FIG. 5C.

FIG. 5D illustrates depositing metal 509 in the shallow portions 505 ofthe plurality of second trenches 503 a to fill the empty space 507 shownin FIG. 5C. In one example, the empty space 507 in FIG. 5C is completelyfilled by metal 509. There is at least a part of the dielectric material508 between metal 509 and the semiconductor material 504 to isolatemetal 509 and the semiconductor material 504 electrically.

The above description of illustrated examples of the invention,including what is described in the Abstract, is not intended to beexhaustive or to limit the invention to the precise forms disclosed.While specific examples of the invention are described herein forillustrative purposes, various modifications are possible within thescope of the invention, as those skilled in the relevant art willrecognize.

These modifications can be made to the invention in light of the abovedetailed description. The terms used in the following claims should notbe construed to limit the invention to the specific examples disclosedin the specification. Rather, the scope of the invention is to bedetermined entirely by the following claims, which are to be construedin accordance with established doctrines of claim interpretation.

What is claimed is:
 1. A method of a pixel array fabrication,comprising: providing a semiconductor material having a first side and asecond side opposite the first side; etching at least two first trenchstructures and at least one second trench structure, wherein alltrenches have same geometric structures and extend from the first sidetowards the second side of the semiconductor material; widening thesecond trench structure to form a shallow portion proximate to the firstside of the semiconductor material, and a deep portion disposed betweenthe shallow portion and the second side of the semiconductor material;depositing a dielectric material within the deep portion and the shallowportion of the second trench structure, and also within the two firsttrench structures; depositing a metal within a region of the shallowportion of the second trench structure such that the dielectric materialwithin the shallow portion is disposed between the metal and thesemiconductor material; disposing a first combination pixel in thesemiconductor material between the first side and the second side of thesemiconductor material, wherein the first combination pixel includes atleast two adjacent first photo sensitive elements configured to receivelight of a first wavelength, and wherein one of the first trenchstructures is disposed between the two adjacent first photo sensitiveelements; disposing a second combination pixel in the semiconductormaterial between the first side and the second side of the semiconductormaterial, wherein the second combination pixel includes at least twoadjacent second photo sensitive elements configured to receive light ofa second wavelength, wherein the other one of the first trenchstructures is disposed between the two adjacent second photo sensitiveelements; and wherein the first combination pixel is adjacent to thesecond combination pixel, and the second trench structure is disposedbetween the first and the second combination pixel.
 2. The method ofclaim 1, wherein the shallow portion of the second trench structuretapers from the first side of the semiconductor material towards thesecond side of the semiconductor material such that a width of theshallow portion of the second trench structure proximate to the firstside of the semiconductor material is greater than a width of the deepportion of the second trench structure proximate to the second side ofthe semiconductor material.
 3. The method of claim 1, wherein at leastpart of the metal deposited within the shallow portion of the secondtrench structure is wider than the deep portion of the second trenchstructure.
 4. The method of claim 1, wherein the metal tapers from thefirst side of the semiconductor material towards the deep portion of thesecond trench structure such that a ray of off-axis incident lightpropagates through the first side of the semiconductor material and isreflected by the metal towards one of the first and second photosensitive elements.
 5. The method of claim 1, wherein the firstwavelength is different from the second wavelength.
 6. The method ofclaim 1, wherein the dielectric material is one of negative chargedielectric materials or one of positive charge dielectric materials.